It is very often necessary to associate data with a motion-image stream. In television so-called ‘metadata’ is frequently associated with one or more fields or frames of analogue or digital video. Typically the data associated with a field or frame describes parameters of that field or frame. Examples of parameters that can be carried by metadata include: identification of the video; timecode or frame number; the intended display aspect-ratio; or, a ‘signature’ of the field or frame that enables comparison with fields or frames in another video stream. For convenience, the term “video image” will sometimes be used in this specification to mean either video frame or video field.
The association between the metadata and its respective image is frequently achieved by carrying the metadata in the blanking intervals of television signals, but, because these intervals are discarded by some video processing equipment, the metadata can be lost, or delayed differently from the video itself. This difficulty can be avoided by encoding the metadata within the active area of the video frames; this technique is known as ‘watermarking’. Typical watermarks are intended to be imperceptible to a viewer; known watermarking algorithms include methods that modify the frequency spectrum of the video, and methods that add low amplitude data signals to the video signal.
Although including the metadata within the image frame ensures that the association between the metadata and the video data cannot be lost, modern video processing techniques, especially those which combine spatially-separated and/or temporally-separated pixels, can distort the metadata. As typical metadata carried by watermarking only causes small changes to the values of pixels (because of the requirement for low visibility of the watermark) it may be impossible to decode the metadata after the video has been edited or re-formatted for a different distribution channel.
There is thus a need to carry metadata associated with fields or frames of a motion-image stream in a robust manner that prevents the metadata from being lost or damaged by video processing.